Strontium titanate (STO) is a wide-gap insulator with a perovskite structure that is routinely used as a substrate for growing high-temperature-superconducting cuprates, colossal magnetoresistive manganites and multiferroics, among other materials. Recently, researchers have also discovered a number of interesting properties in various oxide STO heterostructures, including quasi-two-dimensional electron gas (2DEG) behaviour, magnetism, resistance switching, the giant thermoelectric effect and colossal ionic conductivity. Such characteristics mean that these materials might be ideal for all-oxide electronics, thermoelectrics and solid oxide fuel cells.

Two years ago, Yunzhong Chen of the Technical University of Denmark and colleagues reported on the discovery of a new type of heterostructure based on STO that has a conducting interface, despite the fact that it includes a non-crystalline overlayer. The same team is now saying that alumina (Al2O3), one of the most insulating materials in nature, can be grown epitaxially on STO and that the interface produced between these two materials boasts the highest electron mobilities ever observed for complex oxides.

Higher mobilities

“The Hall mobilities of our spinel/perovskite 2DEG exceed 105 cm2/Vs at 2K, which is 100 times the value of those seen in the most widely studied lanthanum aluminium oxide (LAO)/STO system,” Chen told nanotechweb.org. “Such a large increase in electron mobility may be exploited in mesoscopic quantum devices as well as in conventional oxide electronics.”

The team, which also includes researchers from the University of Copenhagen, Leibniz Institute for Solid State and Materials Research, and the Beijing National Laboratory for Condensed Matter Physics, grew γ-alumina on TiO2-terminated STO single crystalline wafers using a technique called pulsed laser deposition. This is one of the most popular physical vapour deposition methods to grow oxide materials. The film growth was monitored in situ by high-pressure reflection high-energy electron diffraction (RHEED). The researchers made use of an α-alumina pellet with a corundum structure as the target to grow the high-quality cube-on-cube γ-alumina on STO.

Chen says that the high electron mobilities in the alumina/STO heterointerface come about thanks to the fact that there is less scattering of electrons by defects and impurities at the spinel/perovskite interface. Another reason is the perfect lattice match at this interface. “Indeed, the oxygen sublattice of α-alumina shows a negligible mismatch (of about 1%) with the STO sublattice,” said Chen. “This is about three times smaller than the mismatch found at the perovskite LAO/STO interface.”

The researchers say that they are now busy working on how to better pattern these oxide interfaces. “If we are lucky, we might see plenty of new physical properties, such as interfacial magnetism and superconductivity, as have already been observed at LAO/STO, and even the quantum Hall effect, in this new γ-alumina/STO heterointerface,” said Chen.

The current work was published in Nature Communications.